[llvm-dev] Assert in register allocation during last chance recoloring

[Quentin Colombet via llvm-dev]

Hi Sanjin,

The spilling/spliting strategy for big registers is known to be weak so I am not surprised you run into this problem.

I’ll have a closer look (sometime this week or next week) and I’ll comment in the PR.

Cheers,
-Quentin

> On Nov 22, 2016, at 1:22 PM, Sanjin Sijaric via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
>  
> I opened the bug #31122 for this issue.
>  
> There are two parts to this problem. Compiled using "llc -debug-only=regalloc test.ll", the test case attached to the bug fails with
>  
> VirtRegMap.h:119: void llvm::VirtRegMap::clearVirt(unsigned int): Assertion `Virt2PhysMap[virtReg] != NO_PHYS_REG && "attempt to clear a not assigned virtual register"' failed.
>  
> The code looks like:
>  
>        %vreg647<def> = COPY %vreg645; QQQQPR_with_dsub_7_then_ssub_0:%vreg647 QQQQPR:%vreg645
>         %vreg657<def> = COPY %vreg655; QQQQPR_with_dsub_7_then_ssub_0:%vreg657 QQQQPR:%vreg655
>         %vreg679<def> = COPY %vreg678; QPR_VFP2:%vreg679 QPR:%vreg678
>         %vreg657:dsub_7_then_ssub_0<def> = VDIVS %vreg679:ssub_2, %vreg647:dsub_7_then_ssub_0, pred:14, pred:%noreg; QQQQPR_with_dsub_7_then_ssub_0:%vreg657,%vreg647 QPR_VFP2:%vreg679
>         %vreg659<def> = COPY %vreg657; QQQQPR:%vreg659 QQQQPR_with_dsub_7_then_ssub_0:%vreg657
>  
> This problem occurs during last chance recoloring, where %vreg679 doesn’t get allocated (and is marked as not spillable), so last chance recoloring kicks in.  It tries to allocate Q0 to %vreg679, where Q0_Q1_Q2_Q3 is assigned to %vreg647, which interferes with %vreg679.  Upon trying to allocate a register to %vreg647, %vreg679 gets evicted from Q0.  But %vreg679 is a fixed register, so it shouldn’t get evicted.  Recoloring is thus deemed successful, and upon returning from tryRecoloringCandidates, we assert when trying to unassign %vreg679.  
>  
> I can get around this by moving the virtual register under consideration in tryLastChanceRecoloring (%vreg679) to RS_Done stage right before the call to tryRecolringCandidates, and then restoring the original stage afterward.  This prevents the eviction of %vreg679 during recoloring.   Alternatively, I can check FixedRegisters in canEvictInterference and return false.
>  
> After the assert is gone, I run into “LLVM ERROR: ran out of registers during register allocation”.  Increasing lcr-max-depth doesn’t help.
>  
> To get past this error, I am thinking about spilling an actual physical register if last chance recoloring fails.  For example,
>  
> P = ….
>  
> virtX = …. Last chance recoloring fails, 
>  
> … = P
>  
> …. = virtX
>  
> will result in something along the lines of
>  
> P = ….
>  
> spill P to fi#0 (original P)
> P = …. (virtX is now in P)
> ….
> Spill P to fi#1 (this is virtX)
> Reload P form from fi#0 (original P)
> … = P
> ….
> Reload P from fi#1 (this is virtX)
> = P
>  
> Does anyone have any other suggestions about dealing with this problem?
>  
> Thank you,
> Sanjin
>  
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